In the past, when producing high strength cold rolled steel sheet, a continuous annealing furnace facility charging inert gas into a furnace atmosphere (see FIG. 11) or a joint cold rolled steel sheet/hot dip galvanized steel sheet facility having such a continuous annealing furnace (see FIG. 12) has been used.
In the continuous annealing furnace facility shown in FIG. 11, in general, cooling using water such as mist cooling and water dip cooling, gas cooling spraying cooled atmosphere gas and/or roll cooling which cools by contact with a cooling medium running through the inside are/is used for the cooling zone.
The joint cold rolled steel sheet/hot dip galvanized steel sheet facility having the continuous annealing furnace shown in FIG. 12 has a plating facility (in the figure, see hot dip galvanization pot 8 and pass line L2). At the time of production of hot dip galvanized steel sheet, the general practice is to maintain the plating adhesion by using gas cooling spraying a cooled atmospheric gas.
Further, in a joint cold rolled steel sheet/hot dip galvanized steel sheet facility having such a continuous annealing furnace, when annealing a non-hot dip galvanized cold rolled steel sheet, as shown by the broken line in FIG. 12, the cold rolled steel sheet passes through a detachable type pass line L1 cut off from the outside air in the same way as another furnace.
In the facility, when annealing mild steel sheet (for example, Si: 0.2% or less), the chemical convertibility did not particularly become a problem.
However, to improve the strength along with the increase in strength of steel sheet due to the need for reducing weight in the automobile field, the amounts of the strength-improving elements Si, Mn, etc. added have been increased. If for example increasing Si to about 1.0%, the steel sheet surface is left with large amounts of Si, Mn, or other oxide films, the chemical convertibility deteriorates, and parts of the Si oxide film not chemically converted, that is, chemical conversion defects called “bald spots”, occur.
In a continuous annealing furnace facility using mist cooling or water dip cooling or another cooling method using water for the cooling zone including part or all of the range of steel sheet temperature of 600 to 250° C. following the heating for recrystallization, the steel sheet surface is exposed to water at the steel sheet temperature, so the steel sheet is pickled or Ni-plated when the steel sheet leaves the annealing furnace.
For this reason, even in high strength cold rolled steel sheet increased in Si or Mn, the chemical convertibility did not particularly become a problem.
However, in a continuous annealing furnace using gas cooling, effusion cooling, and cooling pipe cooling without using a cooling method using water for the cooling method of the cooling zone including part or all of the temperature range or a joint cold rolled steel sheet/hot dip galvanized steel sheet facility having such a continuous annealing furnace, the furnace is filled with an inert atmosphere gas and the oxygen concentration and the dew point are extremely low, so in conventional low Si and Mn materials, the extent of the oxide film does not become a problem. There is normally no facility pickling or Ni-plating steel sheets when it leaves an annealing furnace.
As a result, remarkable deterioration of the chemical convertibility occurs due to the shift to high Si and Mn high strength steel sheet.
In addition, “gas cooling” means the cooling method of cooling by spraying the steel sheet in the furnace by an atmospheric gas of a lower temperature than the steel sheet temperature, “effusion cooling” means the cooling method of cooling by passing the steel sheet through a furnace to which an atmospheric gas of a lower temperature than the steel sheet temperature is supplied, and “cooling pipe cooling” means the cooling method of cooling steel sheet by running a cooling medium through pipes arranged in the furnace and cut off from the furnace atmospheric gas and cooling the furnace atmospheric gas.
Further, a continuous annealing furnace or a joint cold rolled steel sheet/hot dip galvanized steel sheet facility having such a continuous annealing furnace described in the Description includes a continuous annealing furnace of a continuous annealing facility of steel sheet, a continuous annealing furnace of a hot dip galvanization facility of steel sheet, and a continuous annealing furnace of a joint cold rolled steel sheet/hot dip galvanized steel sheet facility.
For this reason, even in a continuous annealing furnace where the cooling method of the cooling zone including part or all of the above-mentioned temperature range is one or more of gas cooling, effusion cooling, and cooling pipe cooling or a joint cold rolled steel sheet/hot dip galvanized steel sheet facility having such a continuous annealing furnace, as shown in FIG. 13 or FIG. 14, the steel sheet is pickled or Ni-plated when it leaves the annealing furnace so as to avoid the formation of “bald spots” and restore the chemical convertibility to the conventional level.
Further, Japanese Patent Publication (A) No. 2006-45615 proposes a method of once oxidizing a steel sheet surface, then reducing it in a reducing atmosphere to prevent deterioration of the chemical convertibility without pickling or Ni-plating after annealing.